Multicast/Broadcast Service (“MBS”) in WiMAX Networks is a service that allows the distribution of data to a group of Mobile Subscribers (“MSs”). IEEE 802.16e standard introduces the notion of MBS_Zone—an area in which multiple Base Stations (“BSs”) synchronously broadcast the same data over the same subchannels at the same time. This technique greatly improves the mobile terminals ability to receive data correctly due to increased energy of the combined signal that each mobile terminal received simultaneously from multiple Base Stations.
MBS Service-Flow (“SF”) carries information to a set of MSs. Typically there are two methods to access a group of MSs for the provisioning of MBS:
Single-BS: Transmission of data over a single Base
Station (“BS”) in the network. The SF is mapped to a Connection Identifier (“CID”) within a specific BS, i.e., the CID is uniquely specified on a “per BS basis”.
Multi-BS: Transmission of data over a plurality of BSs in the network in a synchronized manner. The SF is mapped to a CID unique within a zone at which the service is provided, referred to hereinafter as an MBS_ZONE. The establishment of an MBS connection is typically carried out in a way similar to the way by which unicast connections are established, while the MS registers to the network. This service, the MBS, is maintained regardless of the current mode of the MS (Normal/Sleep/Idle), so that MBS data is transmitted and received regardless of the MS current operation mode.
The Multi-BS access method enables an MS to receive the MBS content, after having successfully registered and the connection established, from several BSs. As explained above, this transmission method requires that the group of BSs participating in the same Multi-BS-MBS service to be synchronized so that data shall be transmitted by all these BSs simultaneously, and to use the same CID and Security Association (“SA”). It should be noted that the MS does not have to be registered at the specific BS from which it receives MBS transmissions.
An MBS_ZONE identifier is used to indicate the group of BSs which use the same CID and SA to distribute an MBS SF. MBS_ZONE can be advertised by the BS in DCD messages, also it can be delivered upon establishment of MBS connection and it can be extracted from the MAP_MBS_IE.
Obviously an MBS_ZONE may include one or more BSs, and a BS may have multiple MBS_ZONE identifiers.
In order to achieve the necessary level of synchronization and allow PHY diversity the downlink data should be identically mapped onto the airburst subchannel-time continuum, a coherent MBS_MAP should be created, and IEEE-802.16e Generic MAC headers, Fragmentation Subheaders, and Packing Subheaders should be applied identically across all the BSs that belong to the same MBS_Zone (see FIG. 1).
However, one of the major drawbacks of the currently known systems is the need to maintain synchronization of MBS downlink (“DL”) flows between the BSs at a level that enables PHY diversity over WiMAX radio links, while the data itself is generated and conveyed along non-synchronized packet networks having high transmission jitter. Furthermore, as no synchronization-related information can be added to these packets, they cannot be used as a source for synchronization information.
One other problem associated with the implementation of the MBS-Zone concept, lies in the fact that each MBS_Zone should be synchronized independently of the other MBS_Zones. However, in case of overlapping MBS_Zones, i.e., a BS that is a member of several MBS_Zones, there is a problem of mutual interference (meaning transmission of different data over the same sub-channels at the same time) which must be avoided. On the other hand, if two Zones do not overlap, it would be beneficial to allow them to transmit different data over the same sub-channels at the same time in order to improve resources' utilization. Thus, a mechanism for assigning time and subcarrier regions for each MBS Zone is required, where such a mechanism takes into account the geographical distribution of the zones, together with the presence or absence of actual data for transmission. The importance of the latter is because when a certain zone has no data to transmit at a certain given time, it will not produce interference and its resources (time and sub-channels) can be reused.